Generally in a pig iron manufacturing process of a blast furnace, as shown in FIG. 1(a), iron ore as a raw material and coke as a fuel are fed through the top of the furnace, while hot air is fed through a tuyere which is formed on a lower portion of the furnace. Thus, the coke is burned, thereby producing pig iron and slag. In accordance with the progress in the pig iron manufacturing process of the blast furnace, at present, expensive coke is replaced with the pulverized coal by using a pulverized coal injecting apparatus 4 in which tuyere is formed to feed the pulverized coal. In the case of a blast furnace in which this pulverized coal is fed as described above, a large cavity which is called a race way (combustion area) 3 is formed on the front of the tuyere due to the high temperature air flow. FIG. 1(b) illustrates in detail the race way 3.
Most of the coke and the pulverized coal are burned in the race way (combustion area) to supply the heat which is required in reducing the ore. However, depending on cases, the unburnt pulverized coal passes through the coke layer within the blast furnace, to be partly discharged to the outside of the furnace, and to be partly accumulated within the coke layer in which the gas velocity is relatively slow. This accumulated unburnt pulverized coal remains within the inner region of the furnace to alter the gas flow. Further, it lowers the intra-furnace temperature, and increases the permeability resistance, with the result that the size of the race way is decreased. As the pulverized coal feeding amount is increased, the decrease of the combustion efficiency of the pulverized coal becomes very serious, with the result that the manufacturing cost of the pig iron is increased.
In order to solve this problem, in the general technique, pure oxygen is enriched, thereby improving the combustion efficiency of the pulverized coal. By carrying out the pure oxygen enrichment through the tuyere, the oxygen concentration in the hot air flow is increased so, as to promote the combustion of the pulverized coal. However, in this method, the flow amount of the hot blast is very large, and therefore, even if oxygen is enriched to a high degree, the actual oxygen concentration increases just by several percent, with the result that the final effect is meager. Further, the cost of newly building the oxygen producing facilities is very high, and therefore, there is a limit in carrying out the oxygen enrichment.
Meanwhile, in order to solve the above described problem, recently, efforts have been concentrated on modifying the structure of the pulverized coal injecting apparatus.
FIG. 2a illustrates one example of this. As shown in this drawing, a pulverized coal injecting apparatus 10 is of a coaxial type, and the pulverized coal is fed through an inner pipe 12, while pure oxygen is enriched in an outer pipe 11. Thus the oxygen concentration is raised to improve the combustion efficiency. In this method, the combustion efficiency is somewhat improved compared with the case of carrying out the hot blast oxygen enrichment. In this method, however, the external oxygen cannot intrude into the pulverized coal flow, but burns only in the outer regions.
FIG. 2b illustrates another effort of solving the above described problem. In this method, an oxygen flow swirler 23 is formed between the coaxial pipes, so as to form a vortex in the inner region of the pulverized coal flow. However, as has been widely recognized, the effect of installing the swirler depends on how suitable it is to the structure of the burner. In other words, if the spiral angle is too deep, the oxygen is directed to the outside of the pulverized coal flow rather than the inner region, with the result that the combustion efficiency is lowered. On the other hand, if these angle is too shallow, it is not different from the case of the general coaxial lance as shown in FIG. 2a.
As still another example of the efforts, there is a single piped expanded pulverized coal injecting apparatus in which the diameter of the single pipe is sufficiently expanded so as to cause a turbulent pulverized coal flow in the leading end of the feeding pipe. In this method, however, there is required a large scale improvement in the auxiliary facilities. Further, if an expanded pipe is installed within the tuyere, the cross sectional area of the tuyere is decreased, thereby impeding the introduction of the hot blast into the blast furnace so as to lower the productivity.
As still another attempt, there is an eccentric double-lance in which two single pipes are set to improve the combustion efficiency. However, if two pulverized coal injecting pipes are installed within a single tuyere, then the cross sectional area is decreased as described above, and therefore, not only are to the productivity and furnace condition stability, adversely effected but also management becomes troublesome since the number of the injecting pipes is doubled.
Besides, in still another attempt, the oxygen feeding angle is altered to forcibly mix the oxygen into the pulverized coal flow. In this case, however, although the combustion efficiency is improved, the flame width is expanded to cause damages in the tuyere. Further, the leading end of the pipe is slightly protruded to alter the feeding angle, and therefore, the protrusion is worn out due to the continuous collisions with the pulverized coal flow.